This invention relates to pressure swing adsorption (PSA) processes, and more particularly to hydrogen production via pressure swing adsorption processes.
The increasing demand for hydrogen, particularly in petroleum refining and processing has provided a strong economic motivation to develop processes to recover hydrogen from refinery fuel gas, coke oven gas and other similar sources as well as from more traditional sources such as reformer off-gas. For most applications, a high purity hydrogen product is required.
The process of production and recovery of hydrogen by steam and/or air reforming of hydrocarbon rich gas streams, such as natural gas, naphtha, or other mixtures of low molecular weight hydrocarbons, is well known in the art. Typical commercial sources for the production of hydrogen include reforming of natural gas or partial oxidation of various hydrocarbons. The reforming is carried out by reacting the hydrocarbon with steam and/or with oxygen-containing gas (e.g., air or oxygen-enriched air), producing a hydrogen gas stream containing accompanying amounts of oxides of carbon, water, residual methane and nitrogen. Unless recovery of carbon monoxide is desired, the carbon monoxide is customarily converted to carbon dioxide by water gas shift reaction to maximize the hydrogen content in the stream. Typically, this gas stream is then sent to a PSA unit. Other hydrogen-rich gas sources that can be upgraded by PSA technology to a high purity product include refinery off-gases with C1-C6 hydrocarbon contaminants. See, e.g., U.S. Pat. No. 3,176,444 to Kiyonaga.
In PSA processes, a multi-component gas is passed to at least one of a plurality of adsorption beds at an elevated pressure to adsorb at least one strongly adsorbed component while at least one relatively weakly adsorbed component passes through. In the case of hydrogen production via pressure swing adsorption (H2 PSA), H2 is the weakly adsorbed component, which passes through the bed. See, e.g., U.S. Pat. No. 3,430,418 to Wagner, U.S. Pat. No. 3,564,816 to Batta and U.S. Pat. No. 3,986,849 to Fuderer et al. At a defined time, the feed step is discontinued and the adsorption bed is depressurized in one or more steps, which permit essentially pure H2 product to exit the bed. Then a countercurrent desorption step is carried out, followed by countercurrent purge and repressurization. H2 PSA vessels generally contain a mixture of activated carbon, for bulk CO2 and CH4 removal, followed by a molecular sieve for CO and N2 removal. See, e.g., U.S. Pat. No. 3,430,418 to Wagner.
Hydrogen production via pressure swing adsorption is a multi-million dollar industry supplying high purity hydrogen for chemical producing industries, metal refining industries and other related industries. The cost of hydrogen from integrated reformer/PSA systems is impacted by both the capital and operating costs of the system. Clearly, economic production of hydrogen requires as low as possible operating and capital costs. Capital cost is largely dictated by the size of the reformer and the size of the PSA beds. PSA bed size decreases as the hydrogen productivity (lb-moles of hydrogen produced/bed volume) of the PSA increases. Hydrogen productivity can be increased by either improved process cycles or improved adsorbents. The size of the reformer is impacted mostly by the hydrogen recovery of the PSA. Improvements in hydrogen recovery in the PSA result in smaller reformer size (as there is a diminished need to produce hydrogen out of the reformer because of better recovery in the PSA). Improvements in hydrogen recovery also result in a reduced demand for reformer feed gas, i.e., natural gas, which generally constitutes the largest operating cost of the reformer. Hydrogen recovery in the PSA can also be improved by either improved process cycles or improved adsorbents.
There are a number of patents describing the use of structured adsorbents for gas separation by pressure swing adsorption. However, these patents do not address the use of self-supported adsorbents for separation of gases other than air but rather of an adsorbent mass containing the active adsorbent as well as a support and/or spacing system.
U.S. Pat. No. 4,234,326 to Bailey et al. relates to the use of activated carbon cloth in adsorptive filters for air purification. The patent states that the advantage of using a flexible charcoal cloth is that it offers a much lower resistance to gas flow compared to a granular adsorbent and a comparable adsorptive capacity. The patent describes a filter comprising layers of charcoal fabric arranged in various ways to accommodate different flow configurations, but preferentially positioned parallel to the direction of the gas flow. The patent also discloses the need to separate the adsorbent fabric layers by air-permeable layers of glass fiber, wool fiber or open cell foam, with a thickness between 0.1 and 1 mm. However, the patent does not address the use of such an adsorbent cloth in a cyclic adsorptive process and does not teach the benefits of fast mass transfer in a fast cycle adsorption process. Further, this adsorptive filter is not regenerated, but disposed of after it becomes spent.
U.S. Pat. Nos. 6,176,897; 6,056,804; 6,051,050; 5,256,172; 5,096,469; 5,082,473; 4,968,329; 4,801,308 and 4,702,903 to Keefer et al. disclose rapid pressure swing adsorption devices for gas separation, which require the use of adsorbent sheets. However, the adsorbent sheets depicted in the Keefer et al. patents always consist of an adsorbent material with a reinforcement material and with spacers between the sheets to establish flow channels in a flow direction tangential to the sheets and between adjacent pairs of sheets. The adsorbent sheets described can be made in various configurations (rectangular, annular stack, spiral-wound, etc.) but always include a support for the adsorbent in the form of an aluminum foil, a metallic mesh, or a matrix which can be woven, non-woven, ceramic or wool. The Keefer et al. patents do not teach the use of a self-supported adsorbent in separation of hydrogen.
U.S. Pat. No. 5,338,450 to Maurer describes the apparatus used in a thermal swing adsorption (TSA) system for gas purification. The apparatus consists of a cylinder containing a spirally wound adsorbent bed. The fluid streams to be treated and recovered after treatment in the bed circulate radially through the adsorbent layers. The adsorbent layers are comprised of adsorbent particles separated by inlet and outlet screens. An impermeable wall is also wrapped between the inlet and outlet screens defining an inlet and an outlet channel between said wall and, respectively, the inlet and the outlet screen for, respectively, distributing and collecting the fluid streams. The patent teaches that since the gas is circulated radially, through the thickness of the adsorbent layers, screens are necessary to retain and form the layers, and an impermeable wall is required to create the channels for fluid circulation.
U.S. Pat. No. 5,389,350 to Freeman et al. describes the manufacture of fibrous activated carbon material from polyarylamides carbonization, the final adsorbent product being in various possible forms including woven or knitted cloth or non-woven fabric. The patent teaches the use of those adsorbing materials for CO2 adsorption and particularly CO2 removal from air. However, rather than emphasizing the use of those adsorbents in a self-supported fashion and describing the physical properties of the materials, which would benefit a PSA gas separation process and their integration the disclosed system, the patent provides a detailed description of the various steps of their manufacturing process.
The earlier patent literature contains a large number of patents describing conventional pressure swing adsorption cycle processes for gas separation where the cycle time is on the order of several minutes. U.S. Pat. No. 3,430,418 to Wagner teaches the use of a four-bed PSA process for hydrogen purification with a total cycle time of 32 or 48 minutes, wherein the on-line time or feed time is a quarter of the total cycle time. U.S. Pat. No. 3,564,816 to Batta teaches the use of a four-bed PSA process for air separation at a cycle time of 12 or 24 minutes and for hydrogen/nitrogen separation at a cycle time of 28 minutes. In a more recent U.S. Pat. No. 5,250,088 by Yamaguchi et al., a four-bed PSA system is operated at a cycle time of 15 or 30 minutes for hydrogen purification. Clearly, the cycle time of PSA systems using conventional adsorbents beads or granules is usually at least several minutes.
In the more recent patents relating the art of rapid pressure swing adsorption, the cycle time is much shorter, on the order of seconds or even fractions of a second. U.S. Pat. No. 6,231,644 to Jain et al. describes an improved air separation process utilizing a monolithic adsorbent material where the cycle time is 35 seconds. U.S. Pat. No. 6,176,897 B1 to Keefer et al. and U.S. Pat. No. 6,056,804 to Keefer et al. claim the operation of ultra rapid PSA system using adsorbent laminate modules at cyclic frequency of 100 cycles per minute which corresponds to a cycle time of 0.6 second and even possibly as high as 600 cycles per minute (0.1 second cycle time). Those patents illustrate the new generation of rapid pressure adsorption systems, operating at very short cycle times and necessitating novel adsorbent configurations, process cycle and mechanical device innovations.
Accordingly, it is desired to provide an improved system for rapid PSA. It is further desired to provide such systems comprising the use of improved adsorbents.
All references cited herein are incorporated herein by reference in their entireties.
Accordingly, the invention provides a pressure swing adsorption process for recovering a product gas from a feed gas, said process comprising:
supplying a pressure swing adsorption apparatus comprising an adsorbent fabric, wherein said adsorbent fabric is self-supported, has an average pore diameter greater than 5 xc3x85 and has a carbon dioxide mass transfer coefficient of at least 0.5 secxe2x88x921;
feeding a feed gas into said pressure swing adsorption apparatus during a feed period not exceeding 100 seconds; and
recovering said product gas from said pressure swing adsorption apparatus.
Also provided is a pressure swing adsorption process for recovering a product gas from a feed gas, said process comprising:
supplying a pressure swing adsorption apparatus comprising an adsorbent fabric, wherein said adsorbent fabric is self-supported and has a carbon dioxide mass transfer coefficient of at least 0.5 secxe2x88x921;
feeding a feed gas into said pressure swing adsorption apparatus during a feed period not exceeding 100 seconds; and
recovering said product gas from said pressure swing adsorption apparatus, wherein said product gas consists essentially of hydrogen.